AchievementManager: Return by reference from GetLock()

This makes the API a little nicer to use, since you don't need to do
a mandatory dereference when passing the lock into any kind of scope
guard.

Source/Core/DolphinQt/Achievements/AchievementLeaderboardWidget.cpp

@@ -25,7 +25,7 @@ AchievementLeaderboardWidget::AchievementLeaderboardWidget(QWidget* parent) : QW
   m_common_layout = new QGridLayout();
 
   {
-    std::lock_guard lg{*AchievementManager::GetInstance().GetLock()};
+    std::lock_guard lg{AchievementManager::GetInstance().GetLock()};
     UpdateData();
   }